In the last post we talked about piping, but we didn't get into what type of pipe you should choose.
"So what pipe should I get for my compressed air system?"
The answer is, "It depends." There is no one size fits all answer. You need to look at the pressure, temperature, flow, the ambient conditions, the quality of air needed, budget, and plans for expansion. Many different types of pipe are available, and there's a reason that each one is available.So let's look at each point:
1. Pressure and Temperature.
Every stick of pipe and every pipe fitting has a max pressure rating. Actually I should say it has a max pressure curve. The temperature of the air affects the pressure rating of the pipe. The hotter the air, the lower the pressure it can hold. Your pipe might say it's rated for 200 psi, but at what temperature?
Most pipes have a "Schedule." Schedule is a number given to the pipe to represent it's wall thickness. Thicker pipe can withstand higher pressures. A more in depth explanation can be found here. The most common pipe found in compressed air installations is Schedule 40.
If the pipe has a Schedule, then the fittings will have a class. Your bushing, elbows, unions, and tees all have a separate temperature/pressure rating, and it's nearly always less than what the pipe is rated for.
Let's look at a typical threaded fitting pressure and temperature chart:
http://www.engineeringtoolbox.com/iron-threaded-fittings-d_855.html
A compressor with an aftercooler will have a discharge approximately 20°F above the ambient temperature. At the most it might get 100°F in the compressor room (although I've seen it get up to 120°F), so that means the air coming out of the compressor will be 120°F or less. In that situation, Class 150 fittings are good up to 300 psi.
However, compressors without aftercoolers are very common. Most of the piston compressors you see do not have an aftercooler. The discharge of these compressors can be up to 350°F. If that was the case, then then Class 150 fittings are only good for 150 psi. If you have a compressor that pumps to 175 psi, then the Class 150 fittings could be a hazard.
Pipe is the same way. You must look at the pressure and temperature. Let's look at Kaeser Smartpipe aluminum piping:
You can see that the pipe is good for 232 psi until you get 115°F, and at that point it drops down to 188 psi. This is fine, because the pipe is designed to work with compressors that typically run at a lower pressure than that, which have an aftercooler, and there is commonly a refrigerated dryer in the system, which lowers the temperature of the air further. However if you were using 250 psi, you couldn't use this pipe. If you were coming straight out of a single-stage piston compressor with no aftercooler, you couldn't use that pipe.
Make sure you are looking at the working pressure of the pipe and fittings, not the burst pressure. The working pressure is the constant pressure a pipe or fitting can withstand, while the burst pressure is an extreme that it can handle for a small amount of time.
2. Flow.
You want to make sure that you buy pipe big enough to handle your flow. How long the pipe run is has an affect on what size you use. We talked about this in the last blog post (look at the pressure drop section), so we won't cover that again. However the type of pipe can affect your flow. If corrosion builds up on the inside of your pipe, then you effectively have a pipe that is of a much smaller diameter.
3. Ambient Conditions.
Above we talked about temperature, but that 's not the only ambient condition that can affect your pipe. If you have pipe that's outside, you should want a pipe with good corrosion resistance. If you work with certain chemicals, you should look at the material compatibility chart and see if the pipe and chemical have any incompatibilities. Yes, I know you're not running that chemical through your pipe and it's not dipped it in, but over the years exposure to the fumes will have an impact. Some compressor oils eat away at plastics and rubber. If your pipe system uses those materials, you may want to see if the compressor oil fumes will eat them away over time.
Look at what your pipe is exposed to internally and externally before choosing the material.
4. Air Quality.
How sensitive is your application to contaminants? Are you doing breathing air or medical air? Galvanized and black pipe may have impurities on the inside of the pipe that are not suitable for what you're doing. For medical air, your pipe has to be "bagged and tagged" before you install it. If the purity of your air is a concern, you may want to go with aluminum or stainless steel. Transair Pipe even has a certification that they cleaned the pipe after making it and that it will not contaminate the application by using it.
For most of you reading this, you don't need your air that clean - at that level of cleanliness, it's exponentially cleaner than the air you're breathing - but if you have a high purity application, it's very important to consider. Imagine if you spent all this money on advanced filtration only to have the pipe after the filters contaminate the process.
5. Budget.
The most advanced piping system might be out of your reach financially. The return on investment may not be quick enough for you. Run the numbers before you buy, but don't always look at the initial cost of the pipe. A good piping system will save you money in the long run. Also cheaper piping often takes longer to install so your labor cost might be higher.
For instance when we look at the labor of our professional pipe installers, a job that might take three days with galvanized pipe would only take one day with Transair or Smartpipe. With the modular aluminum pipe, the total cost of the job is less, even though the pipe costs a lot more. Plus the pipe looks nicer and it's tax deductible. Yes, modular aluminum piping systems are tax deductible in the USA, while steel, copper, and iron piping systems are not.
6. Plans for expansion.
Again, we talked about this in the last blog post, so we won't go over it again. A modular piping system is easily expandable. However, you can make your non-modular piping system ready for expansion by making a loop if possible, and by using unions in areas where expansion might be probable.
So let's look at the pros and cons of common materials.
Black Iron
Air Quality: Very bad. Black iron will rust inside and out.
Corrosion Resistance: Horrible. It's probably the worst choice if rust or corrosion is a concern.
Installation: Tough. Black iron is very heavy and you'll need a pipe threader. Installing large black iron pipe is back-breaking work.
Initial Cost: Super cheap. Black iron is cheap, but you'll probably bear the cost later as it corrodes internally. This will run up your electrical bill, destroy your tools or contaminate your process.
Future expansions: Moderate to difficult. The difficulty of expanding your black iron pipe system all depends on how well you planned it out in the first place. If you set it up properly in the first place and looked ahead by putting unions in key places, then it's not that difficult. If you didn't, you might have to cut into the pipe to do any add-ons.
Galvanized
Air Quality: Moderate. It won't corrode as bad as black iron internally, but it still might over years of use. Galvanized pipe does have some internal contaminants. For most applications it's not a factor, but it you have an application where the air must be extremely high quality, then galvanized pipe after the filters can possible contaminate the process. Sometimes the galvanized coating can peel off.
Corrosion Resistance: Moderate. You shouldn't put it near the ocean, but otherwise it holds up pretty well.
Installation: Tough. It's about the same as installing black iron pipe.
Initial Cost: Good. Galvanized pipe is very common for a reason. It's more expensive than black iron, but has considerably less initial cost than the more advance piping systems. Additionally it doesn't have many of air quality and corrosion problems that you have with black iron.
Future expansions: Moderate to difficult. Exactly the same as black iron above.
Copper
Pressure and temperature rating: Good. Copper pipe comes in all of the common pressure and temperature ranges. In extremes of pressure or temperature, you may have to go with stainless steel instead.
Air Quality: Good to Excellent. Clean copper pipe will give you clean air. If you get the "bagged and tagged" version, then it's pre-cleaned and then specially packaged to prevent contamination in storage and shipping.
Corrosion Resistance: Very Good. Copper pipe is very resistant to all but the most corrosive substances.
Installation: Difficult to Easy. Copper pipe requires brazing or welding. Of course if you're an experienced brazer or welder, then it's not that hard. Our most experienced pipe installer thinks copper is easier than threaded pipe, but harder than aluminum.
Initial Cost: Good to Expensive. Slightly more than galvanized. If you get the bagged and tagged versions, then it's expensive. The cost of copper pipe can vary greatly depending of the price of copper.
Future expansions: Difficult. In most cases it's difficult to add-on to your copper pipe system, unless you planned for it on the initial installation.
Stainless Steel
Air Quality: Excellent. You can use stainless steel piping in the applications that require the highest of purities.
Corrosion Resistance: Excellent. Stainless steel pipe is one of the two "go-to" choices in corrosive environments.
Installation: Very Difficult. Stainless steel pipe needs to be installed by trained professionals only. Yes, if you wanted to use stainless for your house or body shop, you could do it yourself, but normally it's not used in those situations. If you've got the money for stainless, you've got the money to hire a professional installer.
Initial Cost: Astronomical. Stainless steel has a high initial cost, and it's expensive to install. Normally if you have stainless steel it's because you have a very high air purity requirement or you're in a very corrosive environment.
Future expansions: Difficult. It's about the same as copper.
Aluminum
Pressure and temperature rating: Limited. As shown above, most aluminum pipe has a pressure limit of 188-250 psi, depending on temperature. You are not going to use aluminum for a ultra-high pressure application or where it could be exposed to extreme temperatures. For the situations it was designed for, it usually is the best choice, but be careful going beyond those limits.
Air Quality: Excellent. You can use aluminum piping in the applications that require the highest of purities.
Corrosion Resistance: Excellent. Aluminum pipe is one of the two "go-to" choices in corrosive environments. It's corrosion resistance is similar to stainless steel.
Installation: Very Easy. Modular aluminum piping is very easy to install. It's very light weight, too, which makes moving it around a breeze.
Initial Cost: Expensive. Aluminum pipe costs a lot when compared to galvanized or black iron, but that cost is usually made up by the amount of labor you save in the installation.
Future expansions: Easy. Modular aluminum pipe was built to be easy to add-on to.
Putting it all together
For most of you galvanized pipe will be what you have now or what you're going to use. It's great for small installations and general usage. You should, however, take a good look at modular aluminum pipe. There are a lot of benefits to it and there is a return on the initial investment. If you have a large compressed air installation under 232 psi, then it should be the first option on your list. The benefits greatly outweigh the higher initial cost.
If you have extreme temperatures, extreme pressures, a corrosive environment, or need air that has a high degree of purity, then this will dictate what pipe you use. You won't have as many choices.
Otherwise, you have options on what type of pipe you can use. Take a little extra time and look at each one and see what's best for your situation.
